The present invention relates to a device for manufacturing an optical preform by means of an internal vapour deposition process, the device comprising an energy source and a hollow substrate tube, wherein the hollow substrate tube has a supply side and a discharge side, the energy source being moveable along a length of the hollow substrate tube, the device further comprising an elongation tube connected to the hollow substrate tube at the discharge side thereof.
Generally, in the field of optical fibres, multiple thin films of glass are deposited on the inside surface of a substrate tube. The substrate tube is hollow to allow internal deposition. The substrate tube may be of glass, for example glass gitartz (SiO2). Glass-forming gases (viz. reactive gases comprising gasses for the forming of glass and optionally precursors to dopants) are introduced into the interior of the substrate tube from one end (called the “supply side” of the substrate tube).
Doped or undoped glass layers (depending on the use of reactive gases with or without one or more precursors to dopants, respectively) are deposited onto the interior surface of the substrate tube. The remaining gases are discharged or removed from the other end of the substrate tube called the “discharge side” of the substrate tube. The removal is optionally carried out by means of a vacuum pump. The vacuum pump has the effect of generating a reduced pressure in the interior of the substrate tube, which reduced pressure generally comprises a pressure value ranging between 5 and 50 mbar, i.e. 500 and 5000 Pascal.
Several types of internal chemical vapour depositions (CVD) are known, vapour axial deposition (VAD), modified chemical vapour deposition (MDVD) and plasma-enhanced chemical vapour deposition (PECVD or PCVD). Plasma-enhanced chemical vapour deposition (PECVD or PCVD) is a process used to deposit thin films from a gas state (vapour) to a solid state on a substrate. Chemical reactions are involved in the process, which occur after creation of a plasma of the reacting gasses.
Generally, the plasma is induced by the use of electromagnetic radiation, preferably microwaves. Usually, electromagnetic radiation from a generator are directed towards an applicator via a waveguide, which applicator surrounds the substrate tube. The applicator couples the electromagnetic radiation into a plasma that is generated inside the substrate tube. The applicator is moved reciprocally in the longitudinal direction of the substrate tube. Thus, the plasma formed, also called the “plasma reaction zone”, is also moved reciprocally. As a result of this movement a thin vitrified silica layer is deposited onto the interior of the substrate tube with every stroke or pass.
The applicator and the substrate tube are generally surrounded by a furnace so as to maintain the substrate tube at a temperature of 900-1300° C. during the deposition process.
Thus, the applicator is moved in translation over the length of the substrate tube within the boundaries of a furnace which surrounds the substrate tube and the applicator reciprocating within the furnace. With this translational movement of the applicator the plasma also moves in the same direction. As the applicator reaches the inner wall of the furnace near one end of the substrate tube, the movement of the applicator is reversed so that it moves to the other end of the substrate tube towards the other inner wall of the furnace. In other words the applicator and thus the plasma is reciprocating between a reversal point at the supply side and a reversal point at the discharge side of the substrate tube. The applicator and thus the plasma travels a back and forth movement along the length of the substrate tube. Each hack and forth movement is call a “pass” or “stroke”. With each pass a thin layer of vitrified silica material is deposited on the inside of the substrate tube.
Normally, a plasma is generated only in a part of the substrate tube, viz. the part that is surrounded by the applicator. The dimensions of the applicator are smaller than the dimensions of the furnace and of the substrate tube. Only at the position of the plasma, the reactive gasses are converted into solid glass and deposited on the inside surface of the substrate tube. Since the plasma reaction zone moves along the length of the substrate tube, glass is deposited more or less evenly along the length of the substrate tube.
When the number of passes increases the cumulative thickness of these thin films, i.e. of the deposited material, increases thus leading to a decrease in the remaining internal diameter of the substrate tube. In other words, the hollow space inside the substrate tube keeps getting smaller with each pass.
During the deposition process, the substrate tube is clamped into a glass worker lathe. The applicator moves reciprocally only over a part of said substrate tube. This has the disadvantage that only a part of said, expensive, substrate tube can be used to prepare optical fibers. In order to overcome said problem, it is know e.g. from the publications below, to attach a piece of lower quality glass tube, e.g. a so-called elongation tube, to at least the discharge side of said substrate tube. This elongates the total length of the tube. The elongation tubes are clamped into the glass working lathe which increases the effective length of the substrate tube that can be used for deposition.
From European patent application EP 1,801,081 in the name of the present applicant, a device is disclosed for manufacturing an optical preform by means of an internal vapour deposition process, wherein an insertion tube is present in the interior of the substrate tube, at the discharge side, wherein the external diameter and the shape of the insertion tube substantially correspond to the internal diameter and the shape of the substrate tube, and wherein the insertion tube extends beyond the substrate tube. In other words, the insertion tube is inserted in the end of the substrate tube.
From Japanese patent application JP 2003-176148 a method of manufacturing a preform of an optical fibre is known, comprising coaxially attaching an exhaust tube to a quartz tube.
From U.S. Pat. No. 4,389,229 a method of fabricating a light guide preform by a modified chemical vapour deposition process is known, wherein undeposited reactants pass through a glass substrate tube and flow into a reactant exhaust system and are carried therethrough by a uniformly flowing reactant-free gas. The reactants pass through an exhaust tube, a reactant collection chamber, through a pressure control apparatus and into a gas scrubber. The pressure within the exhaust system is maintained substantially constant during the process by continuously monitoring the pressure therein and adjusting the pressure control apparatus accordingly.
From European patent application EP 1,988,062 a device and a method for manufacturing an optical preform by means of an internal vapour deposition process are known, comprising an energy source and a substrate tube, which substrate tube has a supply side for supplying glass-forming precursors and a discharge side for discharging components which have not been deposited on the interior of the substrate tube, whilst the energy source is movable along the length of the substrate tube between a reversal point on the supply side and a reversal point on the discharge side.
One drawback of, for example, the Japanese patent application JP 2003-176148 is that glassy material deposited outside the deposition area in an internal vapour deposition process gives raise to mechanical stress build-up in the substrate tube. This mechanical stress may lead to the substrate tube to break during the optical preform production, which is undesirable.
Another drawback of the known devices comprising elongation tubes attached to the discharge side of the substrate tube is that the connection between the elongation tube and the hollow substrate tube is subjected to a mechanical tension during the following collapsing step which might result in cracking of the substrate tube or resulting primary preform which is undesirable.
In the prior art there is additional problem that may lead to cracking of the substrate tube or primary preform, being the presence of soot inside of the elongation tube. When the internal deposition process is completed the substrate tube having deposited layers of glass on the inside surface thereof is removed, often still having a very high temperature, such as e.g. 800-900 degrees Celsius. When this substrate tube is then slightly tilted, a so-called chimney effect arises causing part of the soot to flow into the substrate tube causing pollution of the glass layers. When the soot is not removed, it might cause cracking during the collapsing process due to mechanical stress applied to said elongation tube. This can be overcome by manually removing said soot from said elongation tube prior to tilting it, but this is difficult to do due to the high temperature.
This problem has previously been solved in the prior art by introducing an insertion tube into the substrate tube. This insertion tube “catches” the soot and can be easily removed from the substrate tube before the subsequent collapsing.
A drawback of the known devices in which an insertion tube is inserted into the substrate tube is that this leads to the built up of undesired glassy deposition on the inner surface of the substrate on the longitudinal position adjacent the insertion tube which glassy deposition leads to an increase in crack formation. This will be explained in more detail below.